Injecting a fluid into a borehole ahead of the bit

ABSTRACT

A method and system for introducing a fluid into a borehole, in which there is arranged a tubular drill string including a drill bit, wherein the drill bit is provided with a passageway between the interior of the drill string and the borehole, and with a removable closure element for selectively closing the passageway in a closing position, and wherein there is further provided a fluid injection tool comprising a tool inlet and a tool outlet, the method comprising passing the fluid injection tool through the drill string to the closure element, and using it to remove the closure element from the closing position; passing the fluid injection tool outlet through the passageway, and introducing the fluid into the borehole from the interior of the drill string through fluid injection tool into the borehole.

[0001] The present invention relates to a method and system forintroducing a fluid into a borehole formed in an underground earthformation. The term fluid is used in the specification and in the claimsto refer to any material that can be pumped through a tubular drillstring, for example cement, lost circulation material, or cleaningfluid. The fluid can also include solid particles.

[0002] Lost circulation material is any material, which can be used toblock fractures in underground formations and is generally of a coarsernature.

[0003] The invention relates in particular to introducing such a fluidinto the borehole ahead of the drill bit at the lower end of the drillstring.

[0004] In the course of a drilling operation, in particular whendrilling an oil or gas well, it is on occasion desirable to pump a fluidinto the borehole. For example, when drilling into a fractured or porouszone, it is desired to cure losses and to maintain formation strength byusing cement and/or lost circulation material. Another example issetting a cement plug for abandonment of a well or well section,possibly followed by drilling of a branched well section.

[0005] It is considered highly undesirable to attempt pumping of a fluidof high density or viscosity and/or comprising coarse material throughthe drill string with a drill bit attached. Conventional drill bits suchas polycrystalline diamond cutter (PDC) bits or roller cone bits areprovided with bit nozzles. However, the fluid would need to be forcedthrough the bit nozzles, and there is a high risk for the nozzles toplug up due to the high shear, rapid pressure drop, and small orifice.Nozzles normally comprise a nozzle channel with a nozzle insert, and theorifice could in principle be increased by removing the nozzle insertsfrom the bit. This option is however not seriously contemplated inpractice since it would significantly impair the performance of the bitfor progressing into the formation.

[0006] Therefore, in practice the drill bit is removed from the drillstring and is replaced by a tool with a sufficiently large orifice inorder that fluid can be introduced. To this end the drill string needsto be pulled out of the borehole. In order that the drill string can bepulled, it is often required to first temporarily stabilize the boreholeby introducing lost circulation material. This can be done through portsin the lower part if the drill string above the drill bit that can beopened and closed again, for example arranged in a so-called circulatingsub. Introducing lost circulation material via this route above the bitcan plug the annulus between borehole wall and the lower part of thedrill string including the drill bit, thereby requiring removal of thedrill string and further complicating operations. The pumping of cementthrough the same ports is not a practical option, since there is asignificant risk that the lower part of the drill string including thedrill bit will be cemented in place.

[0007] When the drill string then has been fully pulled up, the drillbit is for example replaced by a cementing stinger, and the drill stringis lowered again in the borehole to the desired depth, whereupon fluidcan be introduced into the borehole. If it is then desired to resumedrilling, the drill string needs to be pulled out of the borehole hole asecond time, so that the drill bit can be mounted again.

[0008] This procedure is time-consuming and therefore uneconomic.Moreover, introducing a fluid such as cement is often needed in asituation where the borehole is unstable, and in such situation it canbe undesirable to pull the drill string out of the borehole.

[0009] It is an object of the present invention to provide a method forintroducing a fluid into a borehole, wherein fluid can be safelyintroduced through the drill string with a drill bit attached at itslower end.

[0010] It is a further object to provide a system for introducing afluid into a borehole which system allows to drill and to introducefluid into the borehole without the need to replace the drill bit.

[0011] To this end there is provided a method for introducing a fluidinto a borehole formed in an underground earth formation, in whichborehole there is arranged a tubular drill string including a drill bitat its lower end, wherein the drill bit is provided with a passagewaybetween the interior of the drill string above the drill bit and theborehole exterior of the drill bit, and with a removable closure elementfor selectively closing the passageway in a closing position, andwherein there is further provided a fluid injection tool comprising atool inlet, and a tool outlet in fluid communication with the toolinlet, which method comprises the steps of:

[0012] passing the fluid injection tool from a position interior of thedrill string to the closure element, and using the fluid injection toolto remove the closure element from the closing position;

[0013] passing the fluid injection tool to a landing position where thetool outlet has passed through the passageway and where the tool inletresides inside the drill string in fluid communication with the interiorof the drill string; and;

[0014] introducing the fluid from the interior of the drill string intothe borehole, wherein the fluid is received by the tool inlet andintroduced into the borehole through the tool outlet.

[0015] The invention is based on the insight, that a drill bit having asufficiently large passageway can, except for drilling, also be used forlowering a fluid injection tool into the borehole ahead of the drillingbit, in order to introduce a fluid into the borehole. In order that thedrill bit can be efficiently used for both operations, the passageway isprovided with a closure element that can be selectively removed from theclosing position by using the fluid injection tool from the inside ofthe drill string.

[0016] During normal drilling operation, drilling fluid is normallyejected from inside the drill string via nozzles provided in the drillbit. With the passageway open it would not be possible to create thehigh-speed jets of drilling fluid through the nozzles, that are neededto carry the drill cuttings away from the drill bit and aid in formationpenetration. Therefore, the closure element is in the closing positionfor normal drilling operation, and preferably the closure element isprovided with cutting elements that form a joint bit face with thecutting elements on the drill bit during drilling operation.

[0017] For introducing fluid into the borehole, the fluid injection toolis lowered through the drill string into the drill bit to the closureelement, in order to remove the closure element from the closingposition. This is preferably done by connecting the fluid injection toolto the closure element. The outlet of the fluid injection tool can thenbe passed through the passageway into the borehole ahead of the drillbit, whereas the tool inlet remains in and in fluid communication withthe interior of the drill string. In this position, which is referred toas landing position, fluid communication is provided between theinterior of the drill string and the borehole exterior of the drill bitvia the passageway. The length of the fluid injection tool and the shapeof the tool outlet can be designed according to the specificapplication, such as introducing a cement, lost circulation material, ora cleaning fluid.

[0018] It will be clear that a drill bit nozzle is not considered apassageway. Preferably, the smallest cross-sectional area along thepassageway is at least 5 cm², more preferably the passageway is arrangedso as to allow a conduit, e.g. a fluid injection tool, of about 2.5 cm(1 inch) diameter to pass through the passageway.

[0019] U.S. Pat. No. 2,169,223 discloses a reamer bit of the fish-tailtype, provided with a central longitudinal passageway. During normaloperation the reamer bit is used to increase the diameter of an existingborehole, the so-called rat hole. For the reaming operation thepassageway is closed from inside the drill string by means of a plug,which can be retrieved to surface by wireline. Thereafter a flushingapparatus may be lowered for flushing out the rat hole.

[0020] German patent application publication No. DE 198 13 087 A1discloses a system for rotary and hammer drilling and for injectiondrilling. The known system comprises concentrical and decoupled outerand inner drill strings, which form a drill bit at the end. The innerdrill string is provided with injection nozzles along its length, andcan be slid out of the outer drill string for high-pressure injectiondrilling, eventually followed by cementing.

[0021] A drill bit having a passageway and a removable closure elementis disclosed in the International Patent Application with publicationnumber WO 00/17488.

[0022] There is further provided a system for drilling and forintroducing a fluid into a borehole in an underground earth formation,which system comprises:

[0023] a tubular drill string having a drill bit at its lower end,wherein the drill bit is provided with a passageway between the interiorof the drill string above the drill bit and the borehole exterior of thedrill bit, and with a removable closure element for selectively closingthe passageway in a closing position; and

[0024] a fluid injection tool comprising a tool inlet and a tool outletin fluid communication with the tool inlet, which fluid injection toolis arranged so that it can pass from a position interior of the drillstring to a landing position where the tool outlet has passed throughthe passageway and where the tool inlet resides inside the drill stringin fluid communication with the interior of the drill string, andwherein the fluid injection tool is provided with means for removing theclosure element from the closing position.

[0025] The means for removing the closure element from the closingposition suitably comprises a connection means for selectivelyconnecting the fluid injection tool from inside the drill string to theclosure element in the closing position.

[0026] The fluid injection tool serves to guide the fluid from thepassageway to the position in the borehole where the fluid is to beintroduced. Depending on the type of fluid to be introduced, the fluidinjection tool and particularly the tool outlet can appropriately bedesigned.

[0027] If the fluid is cement or lost circulation material, the fluidinjection tool suitably has the form of a cementing stinger, which canbe for example up to 100 m long, or more. If the fluid is lostcirculation material, the tool can be much shorter, for example 10-20 m.Examples of lost circulation material include cellophane flakes, walnuthulls, ground calcium carbonate. When a salt saturated drilling mud ispresent in the borehole, even salt can be used.

[0028] The fluid injection tool can in particular have telescopic form,allowing to increase the length during operation. The telescopic formcan be less robust than a conventional stinger, however this form ispossible since the tool is designed to be deployed within the drillstring, wherein it is better protected than a conventional stinger whenlowered into a borehole.

[0029] The fluid can also be a cleaning fluid. The cleaning fluid canfor example be water or brine, but can also comprise acid (e.g. 5%hydrochloric acid or acetic acid), finely suspended particles (e.g.calcium carbonate, hematite), polymers or other chemical agents, mixedwith water and/or oil. A cleaning fluid can for example be used toremove mudcake from the borehole wall, or to clean the face of the drillbit. In that event the tool outlet has the form of jetting nozzles whichare oriented in the desired direction, or possibly rotatably arranged.

[0030] Suitably, the fluid injection tool is further provided with alanding member, which is arranged so as to close the passage through thebit nozzles when the fluid injection tool is in the landing position.The landing member therefore prevents that the bit nozzles becomeplugged when the fluid is introduced from the drill string via thepassageway and the fluid injection tool into the borehole.

[0031] The invention will now be described in more detail and withreference to the drawings, wherein

[0032]FIG. 1 shows schematically a drill bit for use with the presentinvention;

[0033]FIG. 2 shows schematically an embodiment of the invention; and

[0034]FIG. 3 shows schematically a further embodiment of the invention.

[0035] With reference to FIG. 1, basic features of the present inventionwill now be discussed. FIG. 1 shows schematically a longitudinalcross-section of a rotary drill bit 1, which is a suitable embodimentfor use with the present invention. The drill bit 1 is shown in theborehole 2, and is attached to the lower end of a drill string 3 at theupper end of the bit body 6. The bit body 6 of the drill bit 1 has acentral longitudinal passageway 8 providing fluid communication, and inparticular passage for a tool, between the interior 3 a of the drillstring 3 and the borehole 2 exterior of the drill bit 1, as will bepointed out in more detail below. Bit nozzles are arranged in the bitbody 6. Only one nozzle with insert 9 is shown for the sake of clarity.The nozzle 9 is connected to the passageway 8 via the nozzle channel 9a.

[0036] The drill bit 1 is further provided with a removable closureelement 10, which is shown in FIG. 1 in its closing position withrespect to the passageway 8. The closure element 10 of this exampleincludes a central insert section 12 and a latching section 14. Theinsert section 12 is provided with cutting elements 16 at its front end,wherein the cutting elements are arranged so as to form, in the closingposition, a joint bit face together with the cutters 18 at the front endof the bit body 6. The insert section can also be provided with nozzles(not shown). Further, the insert section and the cooperating surface ofthe bit body 6 are shaped suitably so as to allow transmission ofdrilling torque from the drill string 3 and bit body 6 to the insertsection 12.

[0037] The latching section 14, which is fixedly attached to the rearend of the insert section 12, has substantially cylindrical shape andextends into a central longitudinal bore 20 in the bit body 6 withnarrow clearance. The bore 20 forms part of the passageway 8, it alsoprovides fluid communication to nozzles in the insert section 12.

[0038] Via the latching section 14 the closure element 10 is removablyattached to the bit body 6. The latching section 14 of the closureelement 10 comprises a substantially cylindrical outer sleeve 23 whichextends with narrow clearance along the bore 20. A sealing ring 24 isarranged in a groove around the circumference of the outer sleeve 23, toprevent fluid communication along the outer surface of the latchingsection 14. Connected to the lower end of the sleeve 23 is the insertsection 12. The latching section 14 further comprises an inner sleeve 25which slidingly fits into the outer sleeve 23. The inner sleeve 25 isbiased with its upper end 26 against an inward shoulder 28 formed by aninward rim 29 near the upper end of the sleeve 23. The biasing force isexerted by a partly compressed helical spring 30, which pushes the innersleeve 25 away from the insert section 12. At its lower end the innersleeve 25 is provided with an annular recess 32 which is arranged toembrace the upper part of spring 30.

[0039] The outer sleeve 23 is provided with recesses 34 wherein lockingballs 35 are arranged. A locking ball 35 has a larger diameter than thethickness of the wall of the sleeve 23, and each recess 34 is arrangedto hold the respective ball 35 loosely so that it can move a limiteddistance radially in and out of the sleeve 23. Two locking balls 35 areshown in the drawing, however it will be clear that more locking ballscan be arranged.

[0040] In the closing position as shown in FIG. 1 the locking balls 35are pushed radially outwardly by the inner sleeve 25, and register withthe annular recess 36 arranged in the bit body 6 around the bore 20. Inthis way the closure element 10 is locked to the drilling bit 1. Theinner sleeve 25 is further provided with an annular recess 37, which is,in the closing position, longitudinally displaced with respect to therecess 36 in the direction of the drill string 3.

[0041] The inward rim 29 is arranged to cooperate with a connectionmeans 39 at the lower end of a fluid injection tool 40, which connectionmeans 39 serves as a means for removing the closure element from theclosing position. Only the lower part of the fluid injection tool 40 isshown. The connection means 39 is provided with a number of legs 50extending longitudinally downwardly from the circumference of the fluidinjection tool 40. For the sake of clarity only two legs 50 are shown,but it will be clear that more legs can be arranged. Each leg 50 at itslower end is provided with a dog 51, such that the outer diameterdefined by the dogs 51 at position 52 exceeds the outer diameter definedby the legs 50 at position 54, and also exceeds the inner diameter ofthe rim 29. Further, the inner diameter of the rim 29 is preferablylarger or about equal to the outer diameter defined by the legs 50 atposition 54, and the inner diameter of the outer sleeve 23 is smaller orapproximately equal to the outer diameter defined by the dogs 51 atposition 52. Further, the legs 50 are arranged so that they are inwardlyelastically deformable as indicated by the arrows. The outer, loweredges 56 of the dogs 51 and the upper inner circumference 57 of the rim29 are bevelled. It shall be clear that the lower end of the fluidinjection tool 40 including the connection means 39 can form a separateauxiliary tool for removing the closure element. The auxiliary tool canbe so arranged that it can be releasably mounted on the fluid injectiontool.

[0042] The drill bit 1 with the closure element 10 in the closingposition as shown in FIG. 1 has the shape and full functionality of aconventional PDC drill bit and can thus be used for normal drillingoperation in the same way as well known in the art.

[0043] When it is desired to introduce fluid into the borehole 2 belowthe drill bit 1, the drill bit is first positioned a distance above thebottom of the borehole. Then, the closure element 10 can be outwardlyremoved from the closing position in the drill bit 1.

[0044] To this end, the fluid injection tool 40 is lowered from aposition inside the drill string 3 along the passageway 8 in the bitbody 6, until the connection means 39 engages the upper end of upper endof the latching section 14 of the closure element 10. The dogs 51 slideinto the upper rim 29 of the outer sleeve 23. The legs 50 are deformedinwardly so that the dogs can slide fully into the upper rim 29 untilthey engage the upper end 26 of the inner sleeve 25. By further pushingdown, the inner sleeve 25 will be forced to slide down inside the outersleeve 23, further compressing the spring 30. When the space between theupper end 26 of the inner sleeve 25 and the shoulder 28 has become largeenough to let in the dogs 51, the legs 50 snap outwardly, therebylatching the fluid injection tool to the closure element.

[0045] At approximately the same relative position between inner andouter sleeves, where the legs snap outwardly, the recesses 37 registerwith the balls 35, thereby unlatching the closure element 10 from thebit body 6. At further pushing down of the fluid injection tool theclosure element is integrally pushed out of the bore 20.

[0046] When the closure element has been fully pushed out of the bore20, the diameter of the fluid injection tool 40 determines if fluidcommunication through an annular orifice between the outer diameter ofthe auxiliary tool 40 and the bore 20 is possible. Suitably, the fluidinjection tool is so arranged that no such orifice is present or thatfluid communication through the orifice is blocked.

[0047] The injection of fluid into the borehole through the fluidinjection tool will be described in more detail with reference to FIGS.2 and 3. The connection means 39 co-operates with the latching mechanismof the closure element, so that the closure element 10 remains connectedto the fluid injection tool 40 after having been removed from theclosing position. This allows, when it is so desired after the injectionof fluid, that the closure element 10 can easily be returned to theclosing position. This can be done by retracting the fluid injectiontool 40 until the locking balls 35 of the closure element latch againinto the annular recess 36 of the bit body 6, whereupon the connectionmeans 39 can be disconnected from the closure element 10. It will beunderstood, that in certain applications retraction may not be required,for example when it is not desired to continue drilling after fluidinjection. It is therefore possible that the lower end of the fluidinjection tool simply pushes the closure element out or otherwiseremoves the closure element from the closing position, withoutconnecting itself to the closure element.

[0048] Reference is now made to FIG. 2, which shows schematically anembodiment of the invention that is particularly suitable forintroducing cement into the borehole. The embodiment is based on thedrill bit discussed with reference to FIG. 1, and like referencenumerals as in FIG. 1 are used to refer to similar objects. The fluidinjection tool of this embodiment is a cementing tool 60.

[0049] The drill bit 1 connected to the lower end of the drill string 3is shown in the borehole 2. As shown in FIG. 2, the closure element 10has been outwardly removed from the closing position by the cementingtool 60as discussed with reference to FIG. 1. The connection means isalso arranged so as to prevent fluid communication from the interior ofthe conduit 63 to nozzles in the insert section 14.

[0050] The cementing tool 60 further includes a cementing stinger 62.The stinger 62 comprises a substantially cylindrical conduit 63 of about50 m length, wherein tool inlets 65 and tool outlets 66 are arrangednear the upper and lower ends, respectively. The tool outlets have theform of slits arranged around the circumference of the conduit 63. Onetool inlet is arranged at the top of the fluid injection tool so that itcan receive a ball or plug from the drill string, other inlets can alsobe arranged as slits.

[0051] The cementing tool 60 further comprises a landing member 69 whichis annularly fitted around the conduit 63, between the tool inlet 65 andthe tool outlet 66. The landing member has a landing surface 70, whichcooperates with a landing seat 72 of the drill bit, so that the passageof fluid along the channel 9 a to the nozzle 9 is blocked when thelanding member 69 rests on the landing seat 72.

[0052] In the landing position as shown in FIG. 2 the tool inlet 65resides in the passageway 6, and the tool outlet 66 has passed throughthe drill bit and resides in the borehole ahead of the bit.

[0053] A number of swab cups 74 are fitted around the circumference ofthe conduit 63, and prevent fluid flow in the annulus between theconduit 63 and the wall of the passageway past the position of the toolinlet 65. Further, the fluid injection tool 60 is provided with arupture disc or shear disc 75 which closes off the conduit 63 as long asit is not destroyed, with a fishing neck 76 to which a wireline to thesurface can be attached, and with a catcher or landing seat 77 which isarranged so as to catch balls or plugs that are received in the conduit63, without blocking fluid communication between tool inlet 65 and tooloutlet 66.

[0054] The drill bit 1 can for example have an outer diameter of 21.6 cm(8.5 inch), with a passageway of 6.4 cm (2.5 inch). The conduit 63 ofthe fluid injection tool in this case can have an outer diameter of 5.1cm (2 inch).

[0055] During normal operation, the drill bit 1 with the closure element10 in the closing position can be used for drilling in the borehole 2.During drilling, drilling fluid is circulated down the drill string,through the bit nozzles 9 into the borehole 2, and up to the surface,carrying drill cuttings to the surface. It is assumed that the fluidinjection tool is located at the surface in the course of drilling, butit will be clear that the tool can also be stored in the drill stringabove the drill bit.

[0056] We will now consider the situation that a significant loss ofdrilling fluid is noticed, which loss is caused by drilling into afractured/porous formation layer. It is desired to cure the losses byblocking fluid flow into the fractured formation by means of cement.

[0057] Then, rotation of the drill bit is stopped, and if necessary ashort section of the drill string is retrieved, to allow sufficientspace in the borehole ahead of the bit. The cementing tool 60 isdeployed by pumping down or lowered through the drill string 3 bywireline attached to the fishing neck 76. The connection means at thelower end 61 connects to the latching section 14 of the closure element10 and unlatches the closure element from the bit body 6. The closureelement 10 is fully removed from the closing position by further pushingor pumping the fluid injection tool down, until the landing member 69lands on the landing seat 72, where it blanks off the openings to thechannels 9 a.

[0058] Then the rupture disc 75 is destroyed for example by applyingoverpressure, and cement is circulated down in the interior of the drillstring, preceded by a ball or plug if desired. The bottom of the cementreaches the drill bit 1, flows in the passageway 8, where it is receivedby the tool inlets 65, and then passes through the bit and further inthe conduit 63, until it reaches the tool outlets 66. There it isintroduced into the borehole. The ball or plug is caught in the catcher77. The blow-out-preventer of the well may be shut to enable the cementto be squeezed into the formation. When the top of the cement in theannulus between the conduit 63 and the borehole wall about reaches thelevel of the face of the bit body 6, or earlier, pumping of the cementis stopped. The drill string 3 including drill bit 1 and cementing tool60 are raised sufficiently to ensure that the insert section is abovethe cement. The drill string and fluid injection tool are cleaned bycirculating drilling fluid, while the cement is setting. The hardeningof the cement can be tested by setting down the fluid injection tool onthe cement plug.

[0059] When the cement has hardened sufficiently, the fluid injectiontool can be retracted to re-latch the closure element 10 into theclosing position. Then, the cement can be drilled out. If fluid lossesare cured, the fluid injection tool can be retracted to surface, anddrilling can be continued. It shall be clear that the drill bit for usein such a cementing application should preferably be provided with aclosure element that has a significantly smaller diameter than theborehole. In that case the cementing tool can also easily be retractedwithout disturbing the cement setting. In comparison with conventionalcementing, the drill string does not have to be pulled out of theborehole for the entire operation of curing losses, and it is also notnecessary to first stabilize the borehole by lost circulation material.

[0060] The fluid injection tool can further be provided with a means fortreating the cement before introducing it into the borehole, so as toinfluence the hardening process. It is known in the art that additivesto the cement can be used to trigger a reaction under downhole wellconditions which initiates the hardening. The fluid injection tool caninclude a storage tank for additives, which is arranged so thatadditives are mixed to the cement received from the drill string beforethe mixture is introduced into the borehole. It is also possible thatadditives are already contained in encapsulated form in the cementreceived through the drill pipe. In this case, the fluid injection toolcan include a shearing device which breaks up the encapsulated additivesso that they can react with the cement.

[0061] It will be clear, that instead of drilling the cement plug out,the borehole section below the plug can also be abandoned. In the lattercase, drilling can be continued in a deviating direction, or the entiredrill string can be retrieved to surface.

[0062] An embodiment suitable for introducing lost circulation materialahead of the drill bit looks basically similar to the embodimentschematically shown in FIG. 2, wherein the main difference is that thestinger 62 is typically shorter, for example 10-20 m.

[0063] In another application of an embodiment as shown in FIG. 2, thefluid injection tool can be pre-loaded at the surface with fluid,wherein the tool outlet is closed. After lowering the tool to thelanding position, the tool outlet is opened and fluid is pushed orpumped into the borehole. The fluid can for example consist of twoseparate components, which form a polymer or elastomer after they havebeen mixed. If the mixing is done shortly before the mixture isintroduced in the borehole, a polymer plug can be arranged in theborehole, for example a polyurethane plug.

[0064] In yet another application a cement plug is set in the boreholeby contacting two cement-forming fluid components only in the boreholeoutside of the drill string, wherein the first fluid component isintroduced into the borehole through the fluid injection tool asdescribed hereinbefore, and wherein the second component is pumped downthe annulus between borehole and drill string. This is particularlyadvantageous in a situation of acute losses of drilling fluid downhole.On the one hand there may be no time or too high a risk for pulling thedrill string up; on the other hand the losses allow to pump fluid downboth inside and outside the drill string without taking additionalmeasures to prevent excessive pressure build-up. A further advantage isthat one can achieve almost instantaneous setting of the cement aftercontacting of the two cement-forming components, without running therisk of solidifying prematurely in the drill string. In this way theoperational risk and time needed for cementing is further reduced.

[0065] Two-component cement systems are well known in the art, see forexample the book “Well Cementing” by B. E. Nelson, Elsevier Science,1990, Schlumberger Educational Services, TSL-4135/ICN-015572000, section6-11.3 (page 6.13) or the U.S. Pat. Nos. 5,447,197 and 5,547,506.

[0066] Suitably, a cement slurry is introduced as the firstcement-forming component through the fluid injection tool, and contactedwith an aqueous or oil-based second cement-forming component whichtriggers the slurry to set. An aqueous Portland cement slurry as firstcomponent and a diesel oil mixed with Bentonite as second component formupon contact a very viscous cementitious mass. It shall be clear that astwo-component cement systems in this context are also regarded organicand inorganic two-component systems which have the ability to almostinstantaneously form a solid mass when the two components come intocontact, such as two component (epoxy) resins, polyesters, siliconerubbers, and calcium carbonate/sodium silicate.

[0067] Reference is made to FIG. 3, which shows schematically a furtherembodiment of the present invention. This embodiment is suitable forcleaning of the borehole wall ahead of the drill bit. The embodiment ofFIG. 3 is similar to the embodiment shown in FIG. 2, and like referencenumerals as in FIGS. 1 and 2 are used to refer to similar objects. Themain difference to the embodiment of FIG. 2 is that the fluid injectiontool 40 does not comprise a cementing stinger, but rather a jet cleaningtool. The jet-cleaning tool 76 comprises one or more nozzles 78 radiallyarranged in the wall of the conduit 63, wherein the nozzles 78 arerotatably arranged in between two swivels 80, so that rotation isinduced when fluid is injected into the borehole through the nozzleswith pressure.

[0068] The jet cleaning tool can for example be used in conjunction witha drilling operation, in order to remove mudcake from the borehole wall,or to clean a section of a casing above a drilled open hole sectionwhere a liner hanger, packer or other isolating device is to beinstalled. Drilling is stopped, the jet cleaning tool is deployedthrough the drill string and pumped out, thereby unlatching and removingthe closure element from the closing position similar to the unlatchingof the closure element discussed with reference to FIGS. 1 and 2.Cleaning fluid is circulated down the drill string, into the passageway8 of the drill bit 1, where it is received by the tool inlets 65 andguided ahead of the drill bit through the conduit 63. The fluid isintroduced into the borehole via the nozzles 78 at high speed, therebycleaning the borehole wall. Again, there is no need to pull the drillstring out of the borehole for such a cleaning operation.

[0069] In a different embodiment of a jet cleaning tool (not shown) thenozzles can be arranged to eject fluid jets in other directions. Thiscan for example be useful in a situation where the bit face has becomeclogged up (balled) with drill cuttings, so that normal drillingperformance is seriously impaired.

[0070] A shorter version of the jet-cleaning tool, wherein the nozzlesin the landing position point towards the bit face can be used to cleanthe bit face.

[0071] In the embodiments discussed with reference to FIGS. 1-3, theclosure element has been removed from the closing position by fullydetaching the closure element from the bit body. It will be clear,however, that the closure element can be removed from the closingposition in other ways, for example by a pivoting mechanism where theclosure element opens the passageway but remains connected to the drillbit.

1. A method for introducing a fluid into a borehole formed in anunderground earth formation, in which borehole there is arranged atubular drill string including a drill bit at its lower end, wherein thedrill bit is provided with a passageway between the interior of thedrill string above the drill bit and the borehole exterior of the drillbit, and with a removable closure element for selectively closing thepassageway in a closing position, and wherein there is further provideda fluid injection tool comprising a tool inlet, and a tool outlet influid communication with the tool inlet, which method comprises thesteps of: passing the fluid injection tool from a position interior ofthe drill string to the closure element, and using the fluid injectiontool to remove the closure element from the closing position; passingthe fluid injection tool to a landing position where the tool outlet haspassed through the passageway and where the tool inlet resides insidethe drill string in fluid communication with the interior of the drillstring; and introducing the fluid from the interior of the drill stringinto the borehole, wherein the fluid is received by the tool inlet andintroduced into the borehole through the tool outlet.
 2. The methodaccording to claim 1, wherein the fluid injection tool is provided withconnection means for selectively connecting to the closure element, andwherein the step of removing the closure element from the closingposition comprises connecting the fluid injection tool to the closureelement.
 3. The method according to claim 1, wherein the fluid iscement.
 4. The method according to claim 1, wherein the fluid is a firstcement-forming component, and wherein the method further comprisespassing a second cement-forming component down along the annulus betweenthe drill string and the borehole, so as to form cement after the firstand second cement-forming components have come into contact with eachother in the borehole.
 5. The method according to claim 1, wherein thefluid is lost circulation material.
 6. The method according to claim 1,wherein the fluid is a cleaning fluid.
 7. The method according to claim1, wherein the method further comprises the steps of: stopping theinjection of fluid into the borehole; retrieving the fluid injectiontool through the passageway so that the fluid injection tool is fullycontained in the drill string; moving the closure element into theclosing position; and drilling, using the drill bit.
 8. The methodaccording to claim 3, wherein the method further comprises the steps of:stopping the pumping of cement into the borehole; waiting for the cementto harden; retrieving of the fluid injection tool through the passagewayso that the fluid injection tool is fully contained in the drill string;moving the closure element into the closing position; and drilling,using the drill bit.
 9. A system for drilling and for introducing afluid into a borehole in an underground earth formation, the systemcomprising: a tubular drill string having a drill bit at its lower end,wherein the drill bit is provided with a passageway between the interiorof the drill string above the drill bit and the borehole exterior of thedrill bit, and with a removable closure element for selectively closingthe passageway in a closing position; and a fluid injection toolcomprising a tool inlet and a tool outlet in fluid communication withthe tool inlet, which fluid injection tool is arranged so that it canpass from a position interior of the drill string to a landing positionwhere the tool outlet has passed through the passageway and where thetool inlet resides inside the drill string in fluid communication withthe interior of the drill string, and wherein the fluid injection toolis provided with means for removing the closure element from the closingposition.
 10. The system according to claim 9, wherein the means forremoving the closure element from the closing position comprises aconnection means for selectively connecting the fluid injection toolfrom inside the drill string to the closure element in the closingposition.
 11. The system according to claim 9, wherein the drill bitfurther comprises a bit nozzle, and wherein the fluid injection toolcomprises a landing member, which is arranged so as to close off fluidpassage through the bit nozzle when the fluid injection tool is in thelanding position.
 12. The system according to claim 9, wherein the fluidinjection tool comprises a cementing stinger.
 13. The system accordingto claim 12, wherein the cementing stinger comprises means for treatingthe cement before introducing it into the borehole, so as to influencethe cement hardening process.
 14. The system according to claim 9,wherein the fluid injection tool comprises a jet cleaning tool.
 15. Thesystem according to claim 9, wherein the fluid injection tool comprisesa telescopic conduit between tool inlet and tool outlet.
 16. The systemaccording to claim 9, wherein the passageway has a minimumcross-sectional area of at least 5 cm².
 17. The system according toclaim 9, wherein the closure element is provided with cutting elementsthat form a joint bit face with other cutting elements on the bit facewhen the closure element is in the closing position.
 18. The systemaccording to claim 9, wherein the drill bit with the closure element inthe closing position has substantially the shape of a conventional PDCdrill bit or of a conventional roller cone drill bit.
 19. The systemaccording to claims 9, wherein the connection means of the fluidinjection tool is arranged such that it can be disconnected from theclosure element when the closure element has resumed the closingposition after retracting the fluid injection tool from the boreholeinto the drill string.